Energy expenditure and fat metabolism in vivo have not been accurately assessed in patients with Prader-Willi syndrome (PWS). This study aims to determine if daily energy expenditure (EE) and in vivo fat metabolism differ between PWS subjects and normal obese controls by using a state- of-the-art wholeroom indirect calorimeter to measure 24 hr EE, and isotope dilution methods. In addition, activity will be measured in the normal environment using an accelerometer. Infusions of [2H5] glycerol and [1-13C] palmitate will be given to determine rates of lipolysis and free fatty acid esterification (triglyceride synthesis). Body composition will be determined using a variety of techniques (dual energy x-ray absorptiometry, magnetic resonance imaging, and bioelectrical impedence), and the relationship between fat and fat-free mass and energy expenditure will be computed and compared between patients and controls. Adult PWS patients will complete a 14 week treadmill training program, as described in the project on food motivation and behavior. These subjects will have measurements of 24 EE, in vivo fat metabolism, body composition, and activity, pre-and post-training. Fat biopsies will be assessed for fat cell number and size as well as lipoprotein lipase activity. The relationship between these measures in fat and measures of energy expenditure, activity and body composition will be determined. Obese adult controls will have one fat biopsy taken for comparison to patients. Child/adolescent PWS patients and controls will have follow up measurements of 24 hr EE, body composition and activity 10-16 weeks after their initial assessment to determine if these variables change over time and if so, whether these changes can be predicted from any of the initial measures.